A New Interpretation of Madagascar's Megafaunal Decline: the “Subsistence Shift Hypothesis”
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Journal of Human Evolution 130 (2019) 126e140 Contents lists available at ScienceDirect Journal of Human Evolution journal homepage: www.elsevier.com/locate/jhevol A new interpretation of Madagascar's megafaunal decline: The “Subsistence Shift Hypothesis” Laurie R. Godfrey a, *, Nick Scroxton b, c, Brooke E. Crowley d, Stephen J. Burns b, Michael R. Sutherland e, Ventura R. Perez a, Peterson Faina f, David McGee c, Lovasoa Ranivoharimanana f a Department of Anthropology, University of Massachusetts, Amherst, MA 01003, USA b Department of Geosciences, University of Massachusetts, Amherst, MA 01003, USA c Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA d Departments of Geology and Anthropology, University of Cincinnati, Cincinnati, OH 45221, USA e Department of Mathematics and Statistics, University of Massachusetts, Amherst, MA 01003, USA f Departement Bassins Sedimentaires Evolution Conservation (BEC), Universite D'Antananarivo, Antananarivo 101, Madagascar article info abstract Article history: Fundamental disagreements remain regarding the relative importance of climate change and human Received 7 September 2018 activities as triggers for Madagascar's Holocene megafaunal extinction. We use stable isotope data from Accepted 4 March 2019 stalagmites from northwest Madagascar coupled with radiocarbon and butchery records from subfossil bones across the island to investigate relationships between megafaunal decline, climate change, and habitat modification. Archaeological and genetic evidence support human presence by 2000 years Before Keywords: Common Era (BCE). Megafaunal decline was at first slow; it hastened at ~700 Common Era (CE) and Quaternary extinction peaked between 750 and 850 CE, just before a dramatic vegetation transformation in the northwest that Paleoclimate Speleothems resulted in the replacement of C3 woodland habitat with C4 grasslands, during a period of heightened Population expansion monsoonal activity. Cut and chop marks on subfossil lemur bones reveal a shift in primary hunting Indian Ocean trade network targets from larger, now-extinct species prior to ~900 CE, to smaller, still-extant species afterwards. By 1050 CE, megafaunal populations had essentially collapsed. Neither the rapid megafaunal decline beginning ~700 CE, nor the dramatic vegetation transformation in the northwest beginning ~890 CE, was influenced by aridification. However, both roughly coincide with a major transition in human subsistence on the island from hunting/foraging to herding/farming. We offer a new hypothesis, which we call the “Subsistence Shift Hypothesis,” to explain megafaunal decline and extinction in Madagascar. This hypothesis acknowledges the importance of wild-animal hunting by early hunter/foragers, but more critically highlights negative impacts of the shift from hunting/foraging to herding/farming, settlement by new immigrant groups, and the concomitant expansion of the island's human population. The interval between 700 and 900 CE, when the pace of megafaunal decline quickened and peaked, coincided with this economic transition. While early megafaunal decline through hunting may have helped to trigger the transition, there is strong evidence that the economic shift itself hastened the crash of megafaunal populations. © 2019 Elsevier Ltd. All rights reserved. 1. Introduction habitat modification) as triggers for megafaunal extinction con- tinues to be debated (see Battistini, 1965; Dewar, 1984; Burney Whereas it is well known that Madagascar's megafauna et al., 2003, 2004; Crowley, 2010; Douglass and Zinke, 2015; declined and vanished after humans arrived, the relative impor- Burns et al., 2016; Ekblom et al., 2016; Crowley et al., 2017; tance of climate and human impacts (whether through hunting or Salmona et al., 2017; Anderson et al., 2018; Douglass et al., 2018; Hixon et al., 2018). Explanations generally focus on climatic or anthropogenic pressures, or both. Disentangling the effects of * Corresponding author. fi E-mail address: [email protected] (L.R. Godfrey). “natural” and “human-induced” change can be dif cult, especially https://doi.org/10.1016/j.jhevol.2019.03.002 0047-2484/© 2019 Elsevier Ltd. All rights reserved. L.R. Godfrey et al. / Journal of Human Evolution 130 (2019) 126e140 127 since natural aridification and human disturbance (e.g., fire, eight of which we radiocarbon dated (Crowley et al., 2017). We also deforestation, or agriculture) can have similar effects on the checked previously-dated humeri and femora of extinct lemurs in distributions and abundance of endemic plants and animals. Most the collection at the Universite d'Antananarivo for evidence of explanations for the extinction of Madagascar's megafauna posit human butchery. complex sequences of events. The data, however, do not support Our third important new source of data is calcium carbonate the proposed sequences and over the past two decades no cave deposits, or speleothems (such as stalagmites). In recent years, consensus has emerged. there has been an increased interest in investigating Madagascar's The “synergy” hypothesis (Burney et al., 2003, 2004) maintains past climate and habitat transformations using speleothems (Brook that humans were the primary triggers for extinction, although et al.,1999; Burns et al., 2016; Scroxton et al., 2017; Voarintsoa et al., periods of aridification prior to human arrival may have increased 2017). Speleothems provide an underutilized (in Madagascar) high the vulnerability of the megafauna. According to this hypothesis, resolution and precisely dated record of past climate and envi- human arrival around 2400 years ago was followed in rapid suc- ronmental changes (Fairchild and Baker, 2012). Depending on the cession by: (1) megafaunal population crash (due to megafaunal sampling resolution and degree of hydrological averaging in the hunting primarily in more open habitats), (2) the expansion of open karst host rock above the cave, speleothem records can range from habitats (due to a surge in the intensity of natural fires in grassy decadal to subannual in resolution. Recent improvements in mass biomes now uncontrolled by grazers), (3) shrinking forests (due to spectrometry techniques allow for radiometric UeTh disequilib- the impact of fire), and (4) concomitant extinction of forest- rium dating of speleothems with errors frequently less than 1%, dependent large-bodied lemurs and other creatures. The main enabling the construction of very precise chronologies (Hellstrom, inferences of this hypothesis are that the megafaunal crash 2003; Cheng et al., 2013). For example, half of the UeTh dates from occurred largely in the first part of the first millennium CE (within a stalagmite from Anjohibe Cave in northwestern Madagascar have several centuries of human arrival), and that it preceded and in 2s errors that are less than a decade (Scroxton et al., 2017). effect triggered habitat change through fire. Furthermore, it pre- In addition to providing precise, accurate chronologies, speleo- ceded the introduction of many domesticated plants and animals, thems provide continual records of changes in past vegetation and which in turn occurred with the expansion of the trade network rainfall through the use of stable carbon and oxygen isotope ratios and early spread of settlement sites almost 1000 years later. respectively (McDermott, 2004; Lachniet, 2009). Carbon isotope In contrast, Virah-Sawmy et al. (2009, 2010) argued that habitat values recorded in stalagmites can be used to identify major shifts change occurred in synchrony across Madagascar, but not because in vegetation on land overlying caves because, in karst systems, of fire set by humans; rather, it was island-wide drought that isotope signals are transferred from plants to soil waters to spe- triggered both habitat change and the megafaunal crash. According leothem carbonate (reviewed in McDermott, 2004). The d13C values to this hypothesis, the megafauna declined long after humans in stalagmites may be affected by a variety of factors, including the arrived, in association with the peak of a devastating drought. Early amount of isotope discrimination by plants, soil respiration rates, episodes of aridification in the northwest (Wang and Brook, 2013), the degree of air-solution CO2 exchange in the soil and epikarst central highlands (Gasse and Van Campo, 1998), southwest (Mahe (Genty et al., 2001; Wong and Breecker, 2015), and the extent of CO2 and Sourdat, 1972; Burney, 1993; Goodman and Rakotozafy, 1997) degassing at the stalagmite surface. Major changes in the d13C and southeast (Virah-Sawmy et al., 2009, 2010) were not always values of speleothems, however, may also reflect shifts in vegeta- synchronous. However, gradual drying between 50 and 1000 CE tion type (i.e., shifts from forest to grassland), which may or may (1900 and 950 calendar years before present, or cal BP) culminated not be climate-driven (Dorale et al., 1998; Burns et al., 2016). When in an island-wide drought between 950 and 1050 CE that was vegetation is dominated by C3 plants, speleothem carbonates can sufficiently strong to trigger megafaunal extinction (Virah-Sawmy be expected to yield d13C values between 14‰ and 6‰. In À À et al., 2009, 2010). This hypothesis posits that habitat change trig- contrast, carbonates deposited in equilibrium with CO2 respired gered megafaunal extinction, and not the opposite, as is maintained from C plants will generally exhibit d13C values between 6‰ 4 À by the synergy